Solar spectral conversion by a low-cost luminescent coating for greenhouse applications increases crop yield and can contribute to addressing the food crisis. A luminescent coating based on cheap SiO2 particles doped with Eu2+ and Al3+ demonstrated extra photosynthetic active radiation (PAR) in this work. To optimize the efficiency of this phosphor for greenhouse applications, three phosphor series with varying Al/Eu content in SiO2 were synthesized via a sol–gel approach and characterized by luminescence decay time, absorption, luminescent excitation, emission, and quantum yield measurements. With increasing the Eu%, at a fixed Al%, the decay time and quantum yield decreased while the emission shifted to the red. The effect can be explained by a more and more efficient resonance energy transfer to lower energy Eu2+ ions and quenching sites. While increasing the Al% at a fixed Eu%, the decay time and quantum yield increased, and the red-shift was reduced. Both effects can be explained by an enhanced Eu2+ solubility (reduced Eu clustering) through the Al3+ co-doping, causing the average Eu2+-Eu2+ distance to be longer and the onset of concentration quenching to shift to a higher Eu%. Specifically, we found that for 1 mol% Eu2+, a minimum of 4 mol% Al3+ was required to avoid concentration quenching. Two indicators were developed to quantify the UV to PAR converting efficiency and to quantify the PAR transmission enhancement. Both indicators were determined in a real coating sample based on the optimized phosphor. The result showed an additional PAR was provided by our luminescent coating. A general discussion about all factors that can bring the conversion efficiency of a phosphor coating closer to the theoretical maximum will be presented.@en

The broad class of Cu(Al,Ga,In) (S,Se,Te)2 solar absorber materials when doped with Yb3+ are interesting for thin film based luminescent solar concentrator (LSC's) application. In this work the strong and broad absorption properties of co-sputtered CuGaS2 (CGS) thin films combined with the luminescent properties of Yb are reported. Energy-dispersive x-ray spectroscopy (EDS), x-ray diffraction, transmission, excitation, and temperature dependent emission as well as radiative lifetime measurements are performed on thin films with varying Cu:Ga ratios and Yb3+ concentrations. It is found that Yb3+ emission can be broadly sensitized by the host in the range of 200–600 nm. A lower Cu:Ga ratio, crystallinity and post annealing in air provides a positive impact on the sensitization of Yb3+ emission. The temperature dependent time integrated decay curves show a clear thermal energy barrier of about 0.2 eV. Because the exponential tail, with a lifetime of 110 μs, is constant with temperature, we conclude that the barrier is connected to the thermal release of electrons trapped at the Yb2+ ground state. The low energy transfer efficiency from the host to the Yb dopant is attributed to efficient non-radiative electron-hole pair recombination. The prospects and design criteria of Cu(Al,Ga,In) (S,Se,Te)2 solar absorber materials for LSC applications is the further subject of the discussion.@en

The characterization of a wide range of luminescent thin films can be a long and tedious endeavor. With reactive combinatorial sputtering of multiple metal targets, it possible to fabricate thin films with a gradient in composition simply by not rotating the substrate. In this work, combinatorically sputtered thin films of Cr³⁺ and Nd³⁺ doped in the Al₂O₃–Y₂O₃ system (YAlO) are studied for thin film based luminescent solar concentrators (TFLSCs) application. Contrary to mm's thick plate type LSC's, TFLSCs of just several 100 nm thick require much higher Cr³⁺ concentration to achieve 40% absorption which can enable several 10's of W/m² LSC power efficiencies. Our transmission measurements on a Cr₂O₃ film with a thickness gradient result in an absorption cross section at 460 nm of 1.3 ± 0.7 × 10⁻¹⁹ cm² showing that the TFLSC absorption requirement can be fulfilled provided that the Cr³⁺ concentration is in the order of 10²² ions/cm³. The Y:Al ratio of the YAlO host in our films ranged between 0.5 and 3.5, thereby including the monoclinic (Y₄Al₂O₉), perovskite (YAlO₃) and garnet (Y₃Al₅O₁₂) stoichiometry's on a single film. Position dependent XRD, EDX, excitation, emission and lifetime measurements of Cr³⁺ and Nd³⁺ show that the unique gradient film sputtering method is able to characterize thin films as a function of host composition and doping concentration. Energy transfer between Cr³⁺ and Nd³⁺ in co-doped YAlO films is concluded from Cr³⁺ excitation bands observed while monitoring Nd³⁺ emission and from the matching of the rise-time of Nd³⁺ 1340 nm emission (⁴F_3/2 -> ⁴I_11/2) and the decay time of Cr³⁺ 840 nm emission (⁴T₂ -> ⁴A₂). Nd³⁺ lifetime systematically decreases from 0.24 to 0.05 ms with increasing Cr³⁺ concentration in Y₃Al_5-xCr_xO₁₂:Nd (0.05 < x < 2). The constraints of heavily doped Cr³⁺ thin films for enabling adequate absorption and energy transfer to Nd³⁺ in TFLSC applications are the subjects of the discussion.

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The luminescence properties of Tm2+-doped BaCl2 with an orthorhombic structure have been studied as a function of temperature and compared to other Tm2+-doped chlorides. In addition to the 2F5/2 → 2F7/2 (4f13 → 4f13) line emission, two 4f125d1 → 4f13 band emissions are observed at 20 K that can be ascribed to the spin-allowed (3H6,5d1)S=1/2 → 2F7/2 and spin-forbidden (3H6,5d1)S=3/2 → 2F7/2 transitions. So far, the Tm2+ spin-allowed (3H6,5d1)S=1/2 → 2F7/2 transition has only been identified in Tm2+-doped iodides and some bromides but never before in a Tm2+-doped chloride. Its presence in orthorhombic BaCl2:Tm2+ is explained by the absence of a (3H6,5d1)S=1/2 → (3H6,5d1)S=3/2 energy transfer process. As the temperature increases, both 4f125d1 → 4f13 emissions undergo rapid quenching and are no longer observed at 120 K, resulting in an intensity increase of the 4f13 → 4f13 emission. However, above 100 K, the intensity of the 4f13 → 4f13 emission also decreases, most likely due to quenching via (3H6,5d1)S=3/2 → 2F7/2 interband crossing, as enabled by the exceptionally large 4f125d1 Stokes shift.@en

Silicon-aluminum-oxigen (SiAlO) coatings doped with Sm²⁺ and prepared by reactive magnetron co-sputtering of Si, Al, and Sm targets, are attractive for luminescence solar concentrator applications but suffer from the low absorption between 300 and 600 nm. This article reports that the main cause of low absorption is a high concentration of undesired Sm³⁺. This finding is supported by optical transmittance, photoluminescence emission and excitation characterization, and X-ray photoelectron spectroscopy data of the Sm's 3d^5/2 edge. We present an alternative deposition process for obtaining Sm doped SiAlO layers with enhanced Sm²⁺ absorption by incorporating Sm through the use of multilayer thin-film precursors composed of metallic Sm and SiAlO layers. After thermal post-deposition treatments, diffusion and reaction of the metallic Sm layers with the SiAlO host results in coatings showing the characteristic 5d → 4f transitions of Sm²⁺ in the region between 250 and 600 nm which were not detectable in Sm-doped single layers. This same deposition strategy produces Tm doped SiAlO coatings with Tm²⁺‘s characteristic luminescence at 1132 nm when the SiAlO host is in the mullite composition region. The photoluminescence excitation spectrum of Tm²⁺ is compared to phosphor with similar composition and covers the range between 300 and 700 nm.

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This paper reports the fabrication and characterization of several thulium oxide and nitride thin films grown by reactive magnetron direct-current sputtering. Hysteresis curves of the Tm emission spectra of the sputtering plasma versus the flow of N₂ or O₂ around the Tm-metal target were monitored. Emission spectra of atomic transition lines in the region between 370 and 420 nm were identified to be of neutral thulium. The plasma emission was compared to the hysteresis curves generated by monitoring the sputter rate and target voltage. The nitride films' composition and optical properties were determined by X-Ray Diffraction, and optical transmission spectroscopy. The composition of the oxide films was determined by energy dispersive X-ray spectroscopy. The films are initially amorphous but crystallize after thermal treatment at 800°C. The optical bandgap values obtained using the Tauc method are consistent with what has been previously reported for both Tm₂O₃ and TmN prepared by other methods.

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In this study, we systematically vary the Cl/Br and Br/I ratios in CsCaX₃:Tm²⁺ (X = Cl, Br, I) and hereby gradually shift the positions of the Tm²⁺ 4f¹²5d¹-levels as relative to the two 4f¹³ levels. At low temperatures up to five distinct Tm²⁺ 4f¹²5d¹→4f¹³ emissions and the 4f¹³→4f¹³ emission can be observed. As the temperature increases, most of the 4f¹²5d¹→4f¹³ emissions undergo quenching via multi-phonon relaxation (MPR) and at room temperature only the lowest energy 4f¹²5d¹→4f¹³ and the 4f¹³→4f¹³ emission remains. For all compositions a 4f¹³→4f¹³ risetime phenomenon is then observed whose duration matches the 4f¹²5d¹→4f¹³ decay time. It shows the feeding of the 4f¹³ state after 4f¹²5d¹ excitation. Surprisingly, the feeding time becomes longer from Cl→Br→I, while the related 4f¹²5d¹-4f¹³ energy gap becomes smaller. The temperature dependence of the 4f¹²5d¹→4f¹³ and 4f¹³→4f¹³ emission intensity shows a anticorrelation as earlier observed in other systems and confirms that the feeding process is thermally stimulated. However, the thermally stimulated activation energies that control the feeding process, increase from Cl→Br→I despite our observation that the 4f¹²5d¹-4f¹³ energy gap becomes smaller. An analysis reveals that the unexpected behaviour in risetime and activation energy, as a function of composition, cannot be explained by 4f¹²5d¹→4f¹³ feeding via interband crossing, but more likely via MPR where the electron–phonon coupling strength decreases from Cl→Br→I. No strong relation was found between composition and the quantum efficiency (QE) of the 4f¹³→4f¹³ emission, due to the presence of fluctuations that are likely caused by intrinsic differences in sample quality. Nevertheless, a 4f¹³→4f¹³ QE of up to 70% has been observed and the materials can therefore be used in luminescence solar concentrators.

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The prospect of using Tm²⁺-doped halides for luminescence solar concentrators (LSCs) requires a thorough understanding of the temperature dependent Tm²⁺ excited states dynamics that determines the internal quantum efficiency (QE) and thereby the efficiency of the LSC. In this study we investigated the dynamics in CaX₂:Tm²⁺ (X = Cl, Br, I) by temperature- and time-resolved measurements. At 20 K up to four distinct Tm²⁺ emissions can be observed. Most of these emissions undergo quenching via multi-phonon relaxation below 100 K. At higher temperatures, only the lowest energy 5d–4f emission and the 4f–4f emission remain. Fitting a numerical rate equation model to the data shows that the subsequent quenching of the 5d–4f emission is likely to occur initially via multi-phonon relaxation, whereas at higher temperatures additional quenching via interband crossing becomes thermally activated. At room temperature only the 4f–4f emission remains and the related QE becomes close to 30%. Possible reasons for the quantum efficiency not reaching 100% are provided.

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The parameters governing the performance of a luminescent solar concentrator (LSC) are determined for sputtered thin-films of NaI:Tm²⁺, CaBr₂:Tm²⁺, and CaI₂:Tm²⁺. These parameters are determined by using six gradient thin film material libraries, combinatorially sputtered from metallic and pressed powder targets. These films show strong 4f¹³→4f¹²d¹ absorption of maximally 752 cm⁻¹ at.%⁻¹ for NaI:Tm²⁺, 31 cm⁻¹ at.%⁻¹ for CaBr₂:Tm²⁺, and 473 cm⁻¹ at.%⁻¹ for CaI₂:Tm²⁺. This absorption covers the entire visible spectrum and does not overlap with the infrared 4f-4f emission at 1140 nm. Decay measurements are used to estimate the quantum yields of the thin-films. These quantum yields can be as high as 44 % for NaI:Tm²⁺, when doped with 0.3 at.% Tm. Even at doping percentages as low as 0.3 at.%, the films appear to show luminescence quenching. The concentration-dependent absorption and quantum yield are combined with the index of refraction, resolved from transmission measurements, to simulate the optical efficiency of a thin film Tm²⁺-doped halide LSC. These simulations show that LSCs based on Tm²⁺ can display excellent color rendering indices of up to 99 %, and neutral color temperatures, between 4500K and 6000K. Under optimal conditions, thin-films constrained to a thickness of 10μm and 80 % transmission of the visible spectrum, would be able to display optical efficiencies of 0.71 %. This optical efficiency compares favorably to the maximally achievable 3.5 % under these constraints. This efficiency is largely independent of the size of LSC itself.

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The valence stability parameter (E_Ff), defined as the difference between the charge transfer energy to the host intrinsic Fermi energy, was used as criterion to analyze the capability of different host materials within the SiAlON class to stabilize divalent thulium. Available data on charge transfer energies and optical bandgap values are reviewed for Si₃N₄, SiO₂, AlN, and Al₂O₃. In addition, new data on thin films, collected by our gradient sputter deposition and characterization method on silicon and aluminum nitrides (Si_0.75xAl_1-xN), are reported. These data are sufficient to show that, at least in the nitride subsection of the SiAlON class, divalent thulium is not expected to be stable due to the presence of high E_Ff values. The use of sub-stoichiometric silicon nitride and oxide is also briefly considered.

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Intended as an electricity generating replacement for windows, luminescent solar concentrators (LSCs) are at the borderline of indoor photovoltaics. In urban environments, space to install outdoor PV is limited to just the roof of a building, directly limiting the amount of energy that can be generated. LSCs aim to overcome this limitation by seamlessly integrating PV into the building envelope. This chapter explicates the luminescent phenomena and the working principle behind LSCs. Analytical and discretized techniques for simulating LSCs are presented and provided as downloadable examples. These same techniques show that an LSC can increase the performance of its PV frame with negligible dimming, even at nonperfect conversion efficiencies. This enhancement increases with window size. An overview of the optical properties of 28 state-of-the-art LSCs is presented. These LSCs are evaluated on performance for building-integrated purposes. Simulations show that LSCs with an optical efficiency of more than 2.8% are already possible for nontoxic quantum dot-based LSCs, without compromising on color rendering properties. Next to these state-of-the-art LSCs, a new development in the form of thulium-doped halides is highlighted. These halides are able to absorb the entire visible spectrum without coloration, and could be scaled to efficient LSCs of arbitrary sizes thanks to their lack of self-absorption. As IPV is an emerging application for LSCs, most of the chapter focuses on LSCs for window applications in order to outline the theoretical foundations. In addition to that, developments in LSCs for usage as indoor-only photovoltaic are illustrated with a fully worked-out example of an LSC as tabletop. Next to this IPV simulation, colorful designs that apply LSCs in other ways than just windows are highlighted.

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Combinatorial reactive co-sputtering using Al, Si and Sm targets in an Ar + O ₂ atmosphere, resulted in Sm doped SiAlO thin films with a wide Sm concentration- and Si:Al composition gradient. By combining position dependent EDX spectra and laser excited emission spectra, ternary phase diagrams were constructed that directly show the relation between Sm emission intensity, index of refraction, thickness and composition. Using this approach, the Sm ²⁺ and Sm ³⁺ emission intensity ratio was controlled towards films with predominantly Sm ²⁺ emission, which is most favorable for luminescent solar concentrator (LSC) applications. The optimum Sm ²⁺ efficiency was reached when the Al content was about equal to the Sm content. When the Si:Al ratio decreases, the Sm ²⁺ emission intensity strongly drops to almost zero. However, sputtering without Al resulted in no Sm ²⁺ emission intensity at all. The excitation and emission properties of Sm ²⁺ in the optimized thin films, especially the ratio between the 4f→4f and 5d→4f emission that is sensitively susceptible to the co-ordination polyhedron, closely resembles that of Sm ²⁺ doped crystalline powders with the same composition. This strongly suggest that the Sm ²⁺ ions in our amorphous films are coordinated in the same way. A homogeneous thin film on float glass clearly shows the light concentration effect of the red Sm ²⁺ emission. Due to an unexplained low Sm ²⁺ absorption of our films, even the optimized thin films do not luminesce brightly.

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The concentration dependent luminescence of the SrI₂-TmI₂ system was investigated. For Tm²⁺ concentrations up to 5 mol %, the quantum efficiency (QE) of the ²F_5/2→²F_7/2 emission exhibits a constant value above 50%. The QE drops for higher Tm²⁺ concentrations, partly due to concentration quenching, as evidenced by a decreasing luminescence lifetime of the ²F_5/2→²F_7/2 emission, and partly due to the formation of a second crystal phase with CdCl₂ structure, in which the ²F_5/2→²F_7/2 emission is quenched. The temperature and time dependent relaxation dynamics were studied to identify the origin of the limited QE for Tm²⁺-doping levels below 5 mol %. An anti-correlation between the 5d-4f (³H₆,t_2g)_S=3/2→²F_7/2 and 4f-4f ²F_5/2→²F_7/2 emission intensities was found and rationalised by non-radiative, thermally stimulated, inter-configurational 5d-4f relaxation to the emitting ²F_5/2 level of Tm²⁺. Both, the rise time of the 4f-4f and the decay time of the 5d-4f emission become shorter with increasing temperature. We suggest a similar non-radiative relaxation from the 5d level towards the ²F_7/2 ground state to limit the QE below unity. This route becomes more efficient when the 5d (³H₆,t_2g)_S=3/2 state moves closer to the 4f ²F_5/2 and ²F_7/2 states, which is the case for the CdCl₂ phase with a QE close to zero.

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Building-integrated photovoltaics are drawing much attention to make the built environment self-sufficient in terms of electricity. Luminescent solar concentrators (LSCs) can provide this electricity generation when used as photovoltaic windows. Paramount to an efficient LSC are non-overlapping absorption and emission spectra, to avoid self-absorption. This non-overlap can be achieved by absorbing incoming UV light and emitting in the red to infrared. In this article, we present a technique for optimizing LSCs without self-absorption, using Eu³⁺-doped AlN as a model system. The parameters affecting light absorption, emission and transport are extracted from a combinatorially sputtered gradient material library. This library results from a single deposition, with a gradient in thickness and Eu concentration. AlN:Eu³⁺ absorbs strongly until 450 nm, with a peak solar absorption of 499 cm⁻¹ at%⁻¹ at 350 nm due to a charge transfer band. The strongest emission is at 622 nm, thereby exhibiting no self-absorption. The presented optimization model strikes a balance between concentration quenching and absorptivity of Eu dopants by using the parameters extracted from the material library. For thicker films, concentration quenching can be avoided by using a lower dopant concentration, while still outperforming thinner films due to fast increasing absorption. The results demonstrate that, while AlN:Eu³⁺ itself should only be viewed as a model system, thin films doped with rare earths can yield industry-compatible, high efficiency LSCs because of their high absorption coefficients and lack of self-absorption.

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The temperature dependent luminescence properties of Yb2+ doped in NaI, SrI2 and LaI3 powders, prepared by a fast synthesis method involving the rapid melting and quenching in vacuum are reported. The 4f→5d excitation and emission spectra obtained for SrI2:Yb2+ were found to be in accordance with earlier work validating the synthesis methods. In both NaI and SrI2, doping with Yb2+ leads to 5d→4f emission, whereas in LaI3 no 5d→4f emission was found but instead anomalous emission is observed. In order to understand the observed differences, vacuum referred binding energy (VRBE) schemes were constructed for NaI and LaI3. For this purpose a NaI:Tm3+ sample was prepared, establishing the Tm3+ charge transfer energy at 3,5 eV. The VRBE energy scheme of NaI clearly reveals that the 5d states of Yb2+ and the other divalent lanthanides are well below the conduction band making 5d→4f emission possible. In LaI3 the 5d states are almost 1 eV inside the conduction band, which explains the absence of 5d→4f emission and the presence of the anomalous emission.@en

In recent years, Thulium in its 2 + oxidation state has been identified as candidate dopant in halide hosts for luminescent solar concentrators. Yet, some of its luminescent properties with regard to these applications remain unexplored. In this study we report on the temperature dependent photo-luminescent behaviour of NaCl:Tm²⁺, NaBr:Tm²⁺, and NaI:Tm²⁺. These monohalide materials demonstrate up to five distinct emission peaks which can be attributed to the 4f¹²→4f¹² and 4f¹¹5d¹→4f¹² transitions of Tm²⁺. Their time- and temperature dependent luminescence intensity behaviours are explained by a qualitative model describing the thermally stimulated radiative- and non-radiative relaxation dynamics. The behaviour of Tm²⁺ in these monohalides proves to be similar to earlier reported findings on Tm²⁺-doped trihalide perovskites of the form CsCaX₃ (X = Cl, Br, I), however, the 4f-4f emission is by far the most dominant emission between 10 and 300 K.

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Local property-mapping of gradient thin films of MI2:x% Tm2+ (M=Ca,Sr), where x and film thickness vary, is used to optimize for LSC use. Tm2+ absorbs the entire visible spectrum and emits at 1140 nm.

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SiAlON window coatings are applied on an industrial scale to achieve e.g. scratch-resistance and anti-reflection. Doping these SiAlONs with rare-earths adds luminescent functionality, which could be applied in photovoltaics. By using a combinatorial reactive sputtering approach, an amorphous thin film composition library with a Si:Al ratio from 0.062:1 to 3.375:1 and a Eu doping from 4.8 at% to 26 at% is created. This library uniquely combines high absorption, strong emission and absence of light scattering. By combining position-dependent EDX measurements with transmission and emission spectra, properties like the index of refraction, absorption strength, emission wavelength and decay times of the library can directly be related to the composition. Throughout the library, an index of refraction of 1.63±0.03 is observed, typical for a film with low nitrogen content. The library also shows a large absorption coefficient of 1294±8cm⁻¹at.%⁻¹. Laser-excited emission spectra show that the library has a strong redshift from 500 nm to 550 nm with increasing Al concentration. An increase in Eu concentration also causes a shift of the emission to red. Decay spectra show that a high degree of Si greatly improves the luminescence intensity. These functionalized SiAlON coatings can be of great interest for transparent and scatter-free luminescent solar concentrators applied as windows.

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Understanding the behavior of combinatorially developed luminescent materials requires detailed characterization methods that have been lacking thus far. We developed a device for directly surveying the luminescent properties of thin-film libraries created through combinatorial gradient sputter deposition. Step-scan recorded excitation-, emission- and luminescence decay spectra of a thin-film library were resolved and combined with EDX measurements on the same film, relating composition to luminescent properties. This technique was applied to a single-substrate gradient thin-film library of NaBr_0.73I_0.27 to NaBr_0.09I_0.91, doped with 6.5% to 16.5% Eu²⁺. This gradient film closely followed Vegard's law, with emission fluently shifting from 428 to 439 nm. In comparison, pure NaBr:Eu²⁺ showed emission at 428 nm and NaI:Eu²⁺ at 441 nm. Luminescence decay measurements demonstrated a great degree of concentration quenching in the gradient film. From these measurements we could conclude that an optimized phosphor would most efficiently luminesce when close to NaI:Eu²⁺. This gradient film confirmed that the method presented in this work allows to both study and optimize luminescent behavior in a broad range of host- and dopant systems.

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The phenomenon of self-absorption is by far the largest influential factor in the eficiency of luminescent solar concentrators (LSCs), but also the most challenging one to capture computationally. In this work we present a model using a multiple-generation light transport (MGLT) approach to quantify light transport through single-layer luminescent solar concentrators of arbitrary shape and size. We demonstrate that MGLT offers a significant speed increase over Monte Carlo (raytracing) when optimizing the luminophore concentration in large LSCs and more insight into light transport processes. Our results show that optimizing luminophore concentration in a lab-scale device does not yield an optimal optical efficiency after scaling up to realistically sized windows. Each differently sized LSC therefore has to be optimized individually to obtain maximal efficiency. We show that, for strongly self-absorbing LSCs with a high quantum yield, parasitic self-absorption can turn into a positive effect at very high absorption coeficients. This is due to a combination of increased light trapping and stronger absorption of the incoming sunlight. We conclude that, except for scattering losses, MGLT can compute all aspects in light transport through an LSC accurately and can be used as a design tool for building-integrated photovoltaic elements. This design tool is therefore used to calculate many building-integrated LSC power conversion efficiencies.

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